This application claims the benefit of Japanese Application No. 2001-187956 filed Jun. 21, 2001.
The present invention relates to a magnetic resonance (MR) imaging method and a magnetic resonance imaging (MRI) system. More particularly, the present invention relates to an MR imaging method and an MRI system capable of reconstructing good-quality images.
U.S. Pat. No. 2,898,329 has disclosed an MR imaging method according to which:
(1) data acquisition in steady-state free precession (SSFP) is repeated by sequentially changing a phase for phase encoding until data fv(0) is acquired from views v constituting a k-space;
(2) data acquisition in SSFP is repeated by sequentially changing a phase for phase encoding and alternately shifting the phase of an RF pulse by 180xc2x0, whereby data fv(1) is acquired from views v constituting a k-space;
(3) fv(0) and fv(1) are subjected to addition or subtraction in order to produce data Av that is expressed as follows:
Av=0.5xc3x97Fv(0)+0.5xc3x97Fv(1) or 
Av=0.5xc3x97Fv(0)xe2x88x920.5xc3x97Fv(1); and 
(4) an image is reconstructed based on the produced data Av.
According to the MR imaging method disclosed in U.S. Pat. No. 2,898,329, good-quality images are produced in some cases. However, only poor-quality images (for example, images having band artifacts caused by an inhomogeneous magnetic field) can be produced in other cases.
Therefore, an object of the present invention is to provide an MR imaging method and an MRI system capable of reconstructing good-quality images in cases where any conventional MR imaging method can provide only poor-quality images.
From the first aspect of the present invention, there is provided an MR imaging method having steps described below. That is to say, at the first step (1), data acquisition in steady-state free precession (SSFP) is repeated N times (where N equals the power of 2) by sequentially changing a phase for phase encoding until data fv(k) ranging from data fv(0) to data fv(Nxe2x88x921) is acquired from views v constituting a k-space. At this time, the phase of a radio-frequency (RF) pulse is varied based on an expression of 360xc2x0xc2x7vxc2x7k/N. At the second step (2), if an operator designates Fourier transform (FT) imaging, the data fv(k) is phase-encoded relative to the phases indicated by the RF pulse and then subjected to a Fourier transform. This results in data Fv(n). In contrast, if the operator does not designate Fourier transform imaging, the data fv(k) is regarded as data Fv(n) as it is. At the third step (3), any of at least either of weighted addition and maximum intensity projection (MIP) processing and root-means-square conversion which is selected by the operator is performed on the data Fv(n) in order to produce data Av. At the fourth step (4), an image is reconstructed based on the produced data Av.
According to the MR imaging method provided from the first aspect of the present invention, an operator can designate whether a Fourier transform (FT) should be performed on the data fv(k) relative to each of the phases indicated by the RF pulse. The Fourier transform makes it possible to designate whichever of the free induction decay (FID) component of data and the spin echo or stimulated echo component thereof should be dominant owing to the principle described below.
For example, when N=4, if k=0, the phase of an RF pulse is set to 0 for all times of data acquisition. The polarity of the FID component of data fv(0) agrees with a positive Y direction (the positive direction of a Y axis), while the polarity of the spin echo or stimulated echo component thereof agrees with a negative Y direction (the negative direction of the Y axis). If k=1, the phase of an RF pulse is set sequentially to 0, xcfx80/2, xcfx80, 3xcfx80/2, etc. The polarity of the FID component of data fv(1) agrees with the positive Y direction, while the polarity of the spin echo or stimulated echo component thereof agrees with a positive X direction (the positive direction of the X axis). If k=2, the phase of an RF pulse is set alternately to 0 and xcfx80. The polarity of the FID component of data fv(2) agrees with the positive Y direction, and the polarity of the spin echo or stimulated echo component thereof also agrees with the positive Y direction. If k=3, the phase of an RF pulse is set sequentially to 0, 3xcfx80/2, xcfx80, xcfx80/2, etc. The polarity of the FDI component of data fv(3) agrees with the positive Y direction, while the polarity of the spin echo or stimulated echo component thereof agrees with a negative X direction (the negative direction of the X axis).
Since data Fv(0)=fv(0)+fv(1)+fv(2)+fv(3), the FID components are left intact because the spin echo or stimulated echo components are canceled out due to the above polarities. In reality, a situation disagrees with the ideal. Nevertheless, in the resultant data Fv(0), the FID component thereof is dominant. Moreover, since data Fv(1)=fv(0)xe2x88x92jxc2x7fv(1)xe2x88x92fv(2)+jxc2x7fv(3), the spin echo or stimulated echo components are left intact because the FID components are canceled out due to the above polarities. Consequently, the spin echo or stimulated echo component of the data Fv(1) is dominant. In general, if n in data Fv(n) assumes an odd-numbered value, the FID component is dominant. If n assumes an even-numbered value, the spin echo or stimulated echo component is dominant. Thus, whichever of the FID component and the spin echo or stimulated echo component is dominant can be designated.
Moreover, according to the MR imaging method provided from the first aspect of the present invention, an operator can select processing to be performed on the data Fv(n) from among at least either of weighted addition and MIP and root-mean-square conversion. If weighted addition is performed on the data, whichever of the FID component and the spin echo or stimulated echo component is dominant can be designated. If MIP is performed, a signal-to-noise ratio can be improved. Moreover, If root-mean-square conversion is performed, the signal-to-noise ratio can be improved.
According to the MR imaging method provided from the first aspect of the present invention, processing can be selected from among at least four kinds of processing. In cases where any conventional MR imaging method can produce only poor-quality images, good-quality images may be able to be produced.
Studies made by the present inventor have revealed that: according to the MR imaging method disclosed in the U.S. Pat. No. 2,898,329, if root-mean-square conversion is performed on data fv(0) and fv(1) that represent view images having band artifacts, an image devoid of the band artifacts may be produced. Moreover, if the number of times of repetition N is increased (for example, 8 or more) and a Fourier transform and root-mean-square conversion are selected, a good-quality image is produced in many cases.
From the second aspect of the present invention, there is provided an MR imaging method comprising the steps described below. Namely, at the first step (1), data acquisition in SSFP is repeated N times (where N equals the power of 2) by sequentially changing a phase for phase encoding until data fv(k) ranging from data fv(0) to data fv(Nxe2x88x921) is acquired from views v constituting a k-space. At this time, the phase of an RF pulse is varied based on an expression of 360xc2x0xc2x7vxc2x7k/N. At the second step (2), a Fourier transform is performed on the data fv(k) relative to each of the phases indicated by the RF pulse in order to produce data Fv(n). At the third step (3), any of at least either of weighted addition and MIP processing and root-mean-square conversion selected by an operator is performed on the data Fv(n) in order to produce data Av. At the fourth step (4), an image is reconstructed based on the produced data Av.
According to the MR imaging method provided from the second aspect of the present invention, a Fourier transform is performed on the data fv(k) relative to each of the phases indicated by the RF pulse. The Fourier transform makes it possible to designate whichever of the FID component of the data and the spin echo or stimulated echo component thereof should be dominant. Moreover, an operator can select processing to be performed on the data Fv(n) from among at least either of weighted addition and MIP processing and root-mean-square conversion. If weighted addition is performed, whichever of the FID component of the data and the spin echo or stimulated echo component thereof should be dominant can be designated. If MIP processing is performed, a signal-to-noise ratio can be improved. If root-mean-square conversion is performed, the signal-to-noise ratio can be improved. As mentioned above, either of at least two kinds of processing can be selected. In cases where any conventional MR imaging method can produce only poor-quality images, good-quality images may be able to be produced.
Studies made by the present inventor have revealed that when the number of times of repetition N is increased (for example, 8 or more), if a Fourier transform and root-mean-square conversion are performed, good-quality images are produced in many cases.
From the third aspect of the present invention, there is provided an MR imaging method consisting mainly of the steps described below. Namely, at the first step (1), data acquisition in SSFP is repeated N times (where N equals the power of 2) by sequentially changing a phase for phase encoding until data fv(k) ranging from data fv(0) to data fv(Nxe2x88x921) is acquired from views v constituting a k-space. At this time, the phase of an RF pulse is varied based on an expression of 360xc2x0xc2x7vxc2x7k/N. At the second step (2), the data fv(k) is regarded as data Fv(n) as it is. At the third step (3), any of at least either of weighted addition and MIP processing and root-mean-square conversion selected by an operator is performed on the data Fv(n) in order to produce data Av. At the fourth step (4), an image is reconstructed based on the produced data Av.
According to the MR imaging method provided from the third aspect of the present invention, an operator can select processing to be performed on the data Fv(n) from among at least either of weighted addition and MIP processing and root-mean-square conversion. If weighted addition is performed, whichever of the FID component of the data and the spin echo or stimulated echo component thereof should be dominant can be designated. If MIP processing is performed, a signal-to-noise ratio can be improved. If root-mean-square conversion is performed, the signal-to-noise ratio can be improved. Thus, the operator can select either of at least two kinds of processing. In cases where any conventional MR imaging method can produce only poor-quality images, good-quality images may be able to be produced.
Studies made by the present inventor have revealed that: according to the MR imaging method disclosed in the U.S. Pat. No. 2,898,329, if root-mean-square conversion is performed on data fv(0) and fv(1) that represent view images having band artifacts, an image devoid of the band artifacts may be produced in some cases.
From the fourth aspect of the present invention, there is provided an MR imaging method based on any of the aforesaid MR imaging methods. Specifically, a pulse sequence used to acquire data in SSFT enables concurrent acquisition of an FID signal and an echo. Moreover, each of magnetic field gradients exhibits time-varying strength whose values detected during one repetition time TR is integrated to be 0.
Various pulse sequences be used for data acquisition in SSFT are known. For example, a pulse sequence used for fast imaging employing steady state acquisition (FIESTA) and a pulse sequence used for TrueSSFT are known.
According to the MR imaging method provided from the fourth aspect of the present invention, the pulse sequence used for FIESTA can be adopted.
From the fifth aspect of the present invention, there is provided an MR imaging method based on any of the aforesaid MR imaging methods. Herein, the data Fv(n) resulting from the Fourier transform performed relative to each of the phases indicated by the RF pulse is expressed as follows:
Fv(n)=k=0xcexa3Nxe2x88x921fv(k)xc2x7exp{xe2x88x92jxc2x7n2xcfx80xc2x7k/M}
According to the MR imaging method provided from the fifth aspect of the present invention, the Fourier transform can be performed on the data fv(k) relative to each of the phases indicated by the RF pulse.
From the sixth aspect of the present invention, there is provided an MR imaging method based on any of the aforesaid MR imaging methods. Herein, the data Av resulting from weighted addition performed when N=2 is expressed as follows:
Av=0.5xc3x97Fv(0)+0.5xc3x97Fv(1) 
According to the MR imaging method provided from the sixth aspect of the present invention, the FID component of the data fv(k) can be made dominant.
From the seventh aspect of the present invention, there is provided an MR imaging method based on any of the aforesaid MR imaging methods. Herein, the data Av resulting from weighted addition performed when N=2 is expressed as follows:
xe2x80x83Av=0.5xc3x97Fv(0)xe2x88x920.5xc3x97Fv(1)
According to the MR imaging method provided from the seventh aspect, the spin echo or stimulated echo component of the data fv(k) can be made dominant.
From the eighth aspect of the present invention, there is provided an MR imaging method based on any of the aforesaid MR imaging methods. Herein, the data Av resulting from MIP processing of solving a function max{ } for providing a maximum value is expressed as follows:
Av=max{Fv(0), etc., Fv(Nxe2x88x921)}
According to the MR imaging method provided from the eighth aspect, a signal of the largest magnitude among all N signals is adopted. Therefore, a signal-to-noise ratio is improved in many cases.
From the ninth aspect of the present invention, there is provided an MR imaging method based on any of the aforesaid MR imaging methods. Herein, the data Av resulting from root-mean-square conversion is expressed as follows:
Av={square root over ( )}{(Fv(0)2+ . . . +Fv(Nxe2x88x921)2)/N}
According to the MR imaging method provided from the ninth aspect, N data items are all employed and will not be canceled out. Consequently, a signal-to-noise ratio is improved in many cases.
From the tenth aspect of the present invention, there is provided an MRI system consisting mainly of a transmitter coil, a gradient coil unit, a receiver coil, a scanning means, and a data processing means. The transmitter coils transmits a radio-frequency (RF) pulse. The gradient coil unit applies magnetic field gradients. The receiver coil receives an NMR signal. The scanning means drives the transmitter coil, gradient coil unit, and receiver coil so as to acquire data. The data processing means performs arithmetic operations on acquired data so as to produce an image. Herein, the scanning means repeats data acquisition in steady-state free precession (SSFP) N times (where N denotes the power of 2) by sequentially changing a phase for phase encoding until data fv(k) ranging from data fv(0) to data fv(Nxe2x88x921) is acquired from views v constituting a k-space. At this time, the phase of the RF pulse is varied based on an expression of 360xc2x0xc2x7vxc2x7k/N. If an operator designates Fourier transform imaging, the data processing means performs a Fourier transform on the data fv(k) relative to each of the phases indicated by the RF pulse, and thus produces data Fv(n). If the operator does not designate Fourier transform imaging, the data fv(k) is regarded as the data Fv(n) as it is. Thereafter, any of at least either of weighted addition and maximum intensity projection (MIP) processing and root-mean-square conversion selected by the operator is performed on the data Fv(n) in order to produce data Av. Consequently, an image is reconstructed based on the produced data Av.
In the MRI system provided from the tenth aspect of the present invention, the MR imaging method provided from the first aspect thereof can be implemented preferably.
From the eleventh aspect of the present invention, there is provided an MRI system consisting mainly of a transmitter coil, a gradient coil unit, a receiver coil, a scanning means, and a data processing means. The transmitter coil transmits an RF pulse. The gradient coil unit applies magnetic field gradients. The receiver coil receives an NMR signal. Herein, the scanning means repeats data acquisition in SSFP N times (where N denotes the power of 2) by sequentially changing a phase for phase encoding until data fv(k) ranging from data fv(0) to data fv(Nxe2x88x921) is acquired from views v constituting a k-space. At this time, the phase of the RF pulse is varied based on an expression of 360xc2x0xc2x7vxc2x7k/N. The data processing means performs a Fourier transform on the data fv(k) relative to each of the phases indicated by the RF pulse so as to produce data Fv(n). Thereafter, any of at least either of weighted addition and MIP processing and root-mean-square conversion selected by an operator is performed on the data Fv(n) in order to produce data Av. Consequently, an image is reconstructed based on the produced data Av.
In the MRI system provided from the eleventh aspect of the present invention, the MR imaging method provided from the second aspect thereof can be implemented preferably.
From the twelfth aspect of the present invention, there is provided an MRI system consisting mainly of a transmitter coil, a gradient coil unit, a receiver coil, a scanning means, and a data processing means. The transmitter coil transmits an RF pulse. The gradient coil unit applies magnetic field gradients. The receiver coil receives an NMR signal. Herein, the scanning means repeats data acquisition in SSFP N times (where N denotes the power of 2) by sequentially changing a phase for phase encoding until data fv(k) ranging from data fv(0) to data fv(Nxe2x88x921) is acquired from views v constituting a k-space. At this time, the phase of the RF pulse is varied based on an expression of 360xc2x0xc2x7vxc2x7k/N. The data processing means regards the data fv(k) as data Fv(n) as it is. Thereafter, any of at least either of weighted addition and MIP processing and root-mean-square conversion selected by an operator is performed on the data Fv(n) in order to produce data Av. Consequently, an image is reconstructed based on the data Av.
In the MRI system provided from the twelfth aspect of the present invention, the MR imaging method provided from the third aspect thereof can be implemented preferably.
From the thirteenth aspect of the present invention, there is provided an MRI system based on any of the aforesaid MRI systems. Herein, a pulse sequence used to acquire data in SSFP enables concurrent acquisition of an FID signal and an echo. Moreover, each of magnetic field gradients exhibits time-varying strength whose values detected during one repetition time TR is integrated to be 0.
In the MRI system provided from the thirteenth aspect of the present invention, the MR imaging method provided from the fourth aspect thereof can be implemented preferably.
From the fourteenth aspect of the present invention, there is provided an MRI system based on any of the aforesaid MRI system. Herein, the data Fv(n) resulting from the Fourier transform performed relative to each of the phases indicated by the RF pulse is expressed as follows:
Fv(n)=k=0xcexa3Nxe2x88x921fv(k)xc2x7exp{xe2x88x92jxc2x7nxc2x72xcfx80xc2x7k/N}
In the MRI system provided from the fourteenth aspect of the present invention, the MR imaging method provided from the fifth aspect thereof can be implemented preferably.
From the fifteenth aspect of the present invention, there is provided an MRI system based on any of the aforesaid embodiments. Herein, the data Av resulting from weighted addition performed when N=2 is expressed as follows:
Av=0.5xc3x97Fv(0)+0.5xc3x97Fv(1) 
In the MRI system provided from the fifteenth aspect of the present invention, the MR imaging method provided from the sixth aspect thereof can be implemented preferably.
From the sixteenth aspect of the present invention, there is provided an MRI system based on any of the aforesaid MRI systems. Herein, the data Av resulting from weighted addition performed when N=2 is expressed as follows:
Av=0.5xc3x97Fv(0)xe2x88x920.5xc3x97Fv(1) 
In the MRI system provided from the sixteenth aspect of the present invention, the MR imaging method provided from the seventh aspect thereof can be implemented preferably.
From the seventeenth aspect of the present invention, there is provided an MRI system based on any of the aforesaid MRI systems. Herein, the data Av resulting from MIP processing of solving a function max{ } for obtaining a maximum value is expressed as follows:
Av=max{Fv(0), . . . , Fv(Nxe2x88x921)}
In the MRI system provided from the seventeenth aspect of the present invention, the MR imaging method provided from the eighth aspect thereof can be implemented preferably.
From the eighteenth aspect of the present invention, there is provided an MRI system based on any of the aforesaid MRI systems. Herein, the data Av resulting from root-mean-square conversion is expressed as follows:
Av={square root over ( )}{(Fv(0)2+ . . . +Fv(Nxe2x88x921)2)/N}
In the MRI system provide from the eighteenth aspect of the present invention, the MR imaging method provided from the ninth aspect thereof can be implemented preferably.
Therefore, an MR imaging method and an MRI system in which the present invention is implemented may be able to produce good-quality images devoid of band artifacts.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.